It is previously known that a substantially vertical well bore can be turned with a short radius curved section into an inclined or horizontal well bore by providing a drilling tool which includes a bend section defining a transverse bend axis between a forward drill bit support portion and a trailing motor portion. The bend section of the drilling tool tends to steer the well bore so that it turns to a direction at right angles to a plane containing the bend axis. One particular example of this technique is disclosed in my U.S. Pat. No. 5,265,687. In this patent I also proposed that the bore be continued in a horizontal direction after the curved section is complete by adding shims to the underside of the drilling tool.
A method is disclosed in U.S. Pat. No. 5,215,151 (Smith et al) in which the drilling of a bore hole is effected using continuous coiled tubing which extends from a trailing end on a supply reel at the earth's surface to a leading end within the well bore.
The drilling of well bores using continuous coiled tubing is known conventionally and includes the supply of a drilling fluid which is pumped into the trailing end of the coiled tubing for transmitting the drilling fluid to the leading end of the tubing at the base of the well bore. At the base is provided a drilling tool which includes a drill bit rotatable relative to the drilling tool, the drill bit being driven by a motor usually powered by the flow of the drilling fluid through the drilling tool.
It is also previously known that, when drilling a horizontal bore section, the horizontal direction can be better maintained by slowly rotating the drilling tool with the bend section so that the bend section rotates about the longitudinal axis of the drilling direction at a rate less than that of the drill bit.
The above U.S. patent of Smith discloses a technique of steering the drilling tool to vary the azimuth of the curved bore section by providing an orientation device as a part of the drilling tool. The drilling tool thus comprises an upper part fixed relative to the drill tubing and a lower part including the drill bit and the bend section. A control motor system is provided by which the lower section can be rotated relative to the upper section in indexed steps of controlled predetermined amounts in response to motive force provided from the surface in the form of pulses in the drilling fluid.
A similar arrangement is disclosed in U.S. Pat. No. 5,311,952 of Eddison et al which uses an indexing device that is actuated by mud pulses but this in addition states that the reactive torque from the drill bit assists in effecting the rotation in the indexing direction.
These arrangement are generally satisfactory and have achieved some success but are relatively complex involving signalling from the surface and relatively complex mechanical structures in the drilling tool. It is also necessary to halt the drilling action and to lift the weight off the drill bit during the indexing step and therefore it is not possible to use this technique for slowly rotating the drilling tool while the drilling continues.
More recently designs of slowly rotating down-hole motors are currently being proposed which can also be commanded from the surface to start and stop to control changes in direction. However these have the disadvantages that it is difficult to convey power from the surface and also it is difficult to provide enough torque to turn the complete tool while drilling without putting too much torque on the coiled tubing, as this is susceptible to damage if over torqued.
It has also been proposed to steer the drilling tool by rotating the injector about the axis of the drill string. This acts to rotate the tubing which in turn rotates the drilling tool to the required angle.
In U.S. Pat. No. 5,485,889 of the present inventor issued Jan. 23.sup.rd 1996 is disclosed a drilling system for controlling steering of a downhole drilling tool which includes a shallow bend in the drilling tool and a system for rotating the bent drilling tool slowly about a longitudinal axis of the drilling tool while the drill bit rotates more rapidly. The system further allows the slow rotation to be halted at a predetermined orientation of the bend axis so as to effect a change in drilling direction. The system for rotating and halting this slow rotation comprises a downhole swivel coupling between the drilling tool and the drill string so that torque from the drill bit tends to rotate the drilling tool in the opposite direction. A control device in the form of a hydraulic pump is provided to restrict the amount of torque communicated through the swivel coupling and to halt the swivel coupling as required to control the steering of the drill bit. The patent also mentions in passing that a disk brake could also be used in replacement for the pump with the disk brake being actuated for example by a system similar to an anti-lock braking system of an automobile so that the coupling is allowed to rotate at a controlled rate. However the pump arrangement shown has a number of disadvantages in that it is not practical in a high pressure sanitary environment and it is difficult to provide an arrangement which will allow the passage of the required drilling fluid and electrical connections.